haskell source code formatter
This project's goals roughly are to:
- Always retain the semantics of the source being transformed;
- Be idempotent;
- Support the full GHC-haskell syntax including syntactic extensions
-XCPPwhich is too hard);
- Retain newlines and comments unmodified;
- Be clever about using the available horizontal space while not overflowing the column maximum if it cannot be avoided;
- Be clever about aligning things horizontally (this can be turned off completely however);
- Have linear complexity in the size of the input.
In theory, the core algorithm inside brittany reaches these goals. It is rather clever about making use of horizontal space while still being linear in the size of the input (although the constant factor is not small). See these examples of clever layouting.
But brittany is not finished yet, and there are some open issues that yet require fixing:
- only the module header (imports/exports), type-signatures and function/value bindings are processed; other module elements (data-decls, classes, instances, etc.) are not transformed in any way; this extends to e.g. bindings inside class instance definitions - they won't be touched (yet).
- By using
ghc-exactprintas the parser, brittany supports full GHC including extensions, but some of the less common syntactic elements (even of 2010 haskell) are not handled.
- There are some known issues regarding handling of in-source comments. There are cases where comments are not copied to the output (this will be detected and the user will get an error); there are other cases where comments are moved slightly; there are also cases where comments result in wonky newline insertion (although this should be a purely aesthetic issue.)
Try without Installing
You can paste haskell code over here to test how it gets formatted by brittany. (Rg. privacy: the server does log the size of the input, but not the full input/output of requests.)
Other usage notes
- Supports GHC versions
- included in stackage with lts>=10.0 (or nightlies dating to >=2017-11-15)
- config (file) documentation is lacking.
- some config values can not be configured via commandline yet.
- uses/creates user config file in
~/.config/brittany/config.yaml; also reads (the first)
brittany.yamlfound in current or parent directories.
# optionally: # mkdir brittany # cd brittany # cabal sandbox init cabal install brittany --bindir=$HOME/.cabal/bin # -w $PATH_TO_GHC_8_0
cabal unpack brittany cd brittany-0.11.0.0 # cabal new-configure -w $PATH_TO_GHC_8_0 cabal new-build exe:brittany # and it should be safe to just copy the executable, e.g. cp `find dist-newstyle/ -name brittany -type f | xargs -x ls -t | head -n1` $HOME/.cabal/bin/
stackusing a sufficiently recent stackage snapshot (dated to >= 2017-11-15)
stack install brittany # --resolver lts-10.0
(earlier ltss did not include
brittanyyet, but the repo should contain a
stack.yamlthat works with ghc-8.0.)
on ArchLinux via the brittany AUR package using
aura -A brittany
In this gist I have described a haskell setup that includes a shortcut to run brittany formatting.
brittany to VSCode formatting API. Thanks to @MaxGabriel.
brittany plugin that directly uses the brittany library.
Relevant for any editors that properly support the language-server-protocol.
Neovim / Vim 8
The Neoformat plugin comes with support for brittany built in.
Atom Beautify supports brittany as a formatter for Haskell. Since the default formatter is set to hindent, you will need to change this setting to brittany, after installing the extension.
Default mode of operation: Transform a single module, from
stdout. Can pass one or multiple files as input, and there is a flag to override them in place instead of using
stdout(since 0.9.0.0). So:
brittany # stdin -> stdout brittany mysource.hs # ./mysource.hs -> stdout brittany --write-mode=inplace *.hs # apply formatting to all ./*.hs inplace
For stdin/stdout usage it makes sense to enable certain syntactic extensions by default, i.e. to add something like this to your
brittanyonce to create default):
conf_forward: options_ghc: - -XLambdaCase - -XMultiWayIf - -XGADTs - -XPatternGuards - -XViewPatterns - -XRecursiveDo - -XTupleSections - -XExplicitForAll - -XImplicitParams - -XQuasiQuotes - -XTemplateHaskell - -XBangPatterns
Feature Requests, Contribution, Documentation
This currently is a one-person project in the sense that 90% of the code is written by one person. And (unfortunately) it is not my job to keep improving this project. Please forgive that as a consequence my time to invest on new features is rather limited.
Nonetheless I consider it "in active development" :)
One way of speeding things up is to make your own contributions. There is a good amount of high-level documentation at
Copyright (C) 2016-2018 Lennart Spitzner
This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General Public License, version 3, as published by the Free Software Foundation.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more details.
You should have received a copy of the GNU Affero General Public License along with this program. If not, see http://www.gnu.org/licenses/.